Polycarbonate polymers from bisphenol-A molecules, linked together by carbonate linkages as obtained by condensing bisphenol-A and phosgene, are a well known and very useful class of polymers. One of their defects, however, which has been noted in studies by a number of workers, is the drastic drop to impact resistance of these polymers resulting from thermal treatment at temperatures between 70.degree. C. and the polycarbonate glass transition temperature of about 150.degree. C. For example, such loss of impact resistance is discussed in an article by L. J. Broutman et al. in Polymer Engineering and Science, Vol. 16 of 1976, page 74.
It has been proposed heretofore to modify polycarbonate polymers by introducing polyester units into the polymer chain, in particular polyester units from bisphenol-A ("BPA") and terephthalic acid ("TPA"), together with carbonate units from phosgene (USP 3 169,121 of Feb. 9, 1965 to E. P. Goldberg). A thorough reinvestigation of such copolymers resulted in greatly improved products, especially improved in melt processability and heat deformation temperature or glass transition temperature, T.sub.g. (Allowed U.S. patent application Ser. No. 764,623 of Prevorsek and Kesten filed Feb. 1, 1977 now U.S. Pat. No. 4,156,069 of May 22, 1979). Such copolymers have mol ratios of the BPA:TPA:carbonate moieties in the range 1:0.4:0.6-1:0.65:0.35.